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24 JanSome Alternatives to Liquid Penetrant Testing

There are many non-destructive testing techniques. They’re not all suited to every type of application. For example, electromagnetic testing techniques don’t work on most composite materials, while X-rays and computed tomography are perfect. Similarly, ultrasounds are very versatile, but requires direct contact and appropriate coupling with the surface under test, which is not always possible.

What’s Liquid Penetrant Testing?

One of the most common, low-cost solution (and one of the oldest) to non-destructive testing challenges is liquid penetrant testing (PT), also known as liquid penetrant inspection (LPI) and dye penetrant inspection (DPI). The technique leverages capillary action—the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity—to detect surface-breaking defects.

After applying the penetrant and letting it dwell for a certain period of time, the excess is removed and a developer is applied. The developer draws the penetrant from surface-breaking defects where the penetrant seeped, revealing the presence of defects.

LPI in History

The earliest instance of a surface inspection technique resembling penetrant testing involved rubbing carbon black (a by-product of the incomplete combustion of heavy petroleum products) on glazed pottery. The carbon black would accumulate in surface cracks, making them visible when the excess black was wiped off.

By the end of the 19th century, the railway industry was widely using the oil and whiting method. It involved applying a light oil on the part under test. The oil—just as in modern PT—was allowed to dwell into surface-breaking defects, and then whitewash was applied on the surface to make the defects visible (by providing a high-contrast background and drawing the oil from the surface-breaking defects). Until the 1920s, this testing method was very popular because it yielded visible results that could be understood by anyone.

Aircraft inspection, propelled by the advances during World War II, was the engine behind the evolution of liquid penetrant testing.

Advantages of LPI

It is generally accepted that liquid penetrant testing has the following advantages:

But, perhaps the greatest disadvantage is that over time and despite lower costs (less training, cheaper materials), LPI is little more than a screening tool; you can locate defects and measure their length, but using this technique, it’s impossible to determine the severity of its depth or to monitor the progress of defects over time. It also relegates the technique to a pass/fail evaluation, which can lead to discarding healthy parts and retaining unhealthy parts—which can both prove costly.

So, overall, despite the immediate attractiveness of this low-cost alternative, it possesses several downsides that must be looked at long and hard before dismissing more advanced and potentially more expensive inspection solutions, whether you perform your own inspections or contract them out.

Alternatives to Liquid Penetrant Testing

Eddy Current Testing (ECT)

For companies looking to achieve the most bang for their buck, eddy current testing (ECT) on conductive materials can potentially save time and money. Eddy current testing systems have a higher initial cost, but they generate no chemical waste, require little to no surface preparation, and are faster than LPI in typical applications, despite being user dependent.

Eddy Current Array (ECA)

This offshoot of eddy current testing improves on the technology by using multiplexed arrays of coils arranged in rows (instead of one or two coils), which allows covering a larger area in a single pass. Some advantages of ECA over ECT are obvious:

Finally, perhaps eddy current array’s (and ECT’s) greatest advantage over dye-penetrant testing, is ECA’s ability to be flexible or shaped to specifications, making hard-to-reach areas and complex geometries easier to inspect—something that’s even difficult for PT, at times.

Of course, ECA systems are more expensive than ECT solutions and LPI, but, in some situations, they can prove 3 to 10 times faster, depending on the initial surface conditions, the presence of coatings, and so on. ECA also offers critical data, which allow making better decisions when it comes to key assets.

About the Author

Angélique RAUDE

Angélique is Eddyfi's surface product manager. She has a master’s degree in mechanical engineering and a post-graduate degree in non-destructive testing from the University of Bordeaux. She's worked as the principal project leader at TWI in the United Kingdom. Angélique is part the Eddyfi team based out of Lyon, where she coordinates the development efforts of new surface eddy current array products.

In her free time, Angélique stays active, enjoying biking and dancing.